The present invention relates to a method of depositing a film or polycrystalline material on a substrate, and of preparing a relatively stable suspension used in the deposition method.
In making various types of electrical devices it has been found desirable to deposit on the surface of a substrate a relatively dense film of a material with the film being adhered to the substrate. Although thin films of many materials can be obtained by the well-known techniques of vacuum evaporation or sputtering, some materials cannot be satisfactorily deposited by these techniques. For example, pyroelectric materials, such as triglycine sulfate (TGS), triglycine selenate (TGSe) and the like, used in making pyroelectric detectors cannot be deposited by these techniques because they thermally decompose before they melt or exhibit appreciable vapor pressure. As described in U.S. Pat. No. 3,511,991 to H. P. Beerman, one method which has been suggested for forming a film of these materials is to mix particles of these materials with a binder, such as a plastic and apply a film of the mixture to a substrate. The binder serves to bind the particles to each other and to the substrate. However, a disadvantage of this method is that the binder may have an undesirable adverse effect on the desired properties of the resulting film.
A method which has been developed to form a film of these materials without a binder, and described in U.S. Pat. No. 3,767,462 to R. A. Larrabee, is to form a suspension of particles of the material in a volatile suspending medium which is a relatively poor solvent for the material, such as a mixture of an alcohol and water. The suspension is applied to the surface of the substrate and the suspending medium is evaporated to leave a film of the particles on the substrate. The resulting film can be made denser and more adherent by treating it with the vapors of a suitable good solvent, such as water, and drying it. Although this method results in coherent film of the material on the substrate which is adherent to the substrate and which is substantially free of other materials which may adversely affect the properties of the film, this method does have a problem. It has been found very difficult to form a stable suspension of the particles in the alcohol and water mixture. By a stable suspension it is meant that there is no visible settling of the particles for at least the length of time required to deposit the film.